This would be incredible on the market for us geek-hearted non-tech savvy's

Very nice work!

Just gotta find an I2S interface (or similar comms) on the stock Kettle board and then you could potentially control the temp from the app too

I don't think it exists, there's only one microcontroller on the board leaving little reason for a board-level chip-to-chip comms and the microcontroller in use doesn't have any dedicated I²C/SPI/JTAG/etc pins. The chip could presumably do such a thing in software, but there'd be no reason to do so here except maybe to communicate with the 7seg LED driver (which itself is potted preventing easy access). It might be possible to dump microcode from the µC but there is an option to disable that capability which I suspect they would have implemented.

Taking this much further would either involve intercepting the µC > 7seg driver comms or maybe monitoring 7seg driver > 7seg but that felt like a lot of work for not much reward.

This is absolutely awe inspiring, excellent work my friend, wow.